Russian aircraft don't cross forward because Russia's strategy is to use long range missiles and drones to penetrate deep into Ukraine. Having air superiority doesn't mean flying aircraft in, it means having control over the airspace which Russia does. Ukraine has practically no air force left to speak of, and they are entirely unable to stop Russians hitting deep inside Ukraine with impunity. Russia has been holding back from bombing civilian areas into the ground entirely as a choice as opposed to lack of capability to do so. Saying Russia doesn't have air superiority because they don't shoot down every plane that flies over Ukraine is a bit silly.